1. Field of the Invention
The present invention relates to a fingerprint sensor and package method thereof, and more particularly to a capacitive fingerprint sensor and package method thereof.
2. Description of the Prior Art
The principle of capacitive fingerprint sensor is to detect a capacitance variation between ridges and valleys of human finger. Normal depth of the valley is around 20-35 μm with air filled which has dielectric constant of 1. The dielectric constant of human tissue is about 4-8. Therefore minute capacitance variation can be detected by the capacitive fingerprint sensor. Package requirements for capacitive fingerprint sensor are: flat surface with wear resistance, ESD toleration and stiffness.
FIG. 1 shows a cross sectional view of conventional package for capacitive fingerprint sensor. A capacitive sensor chip 120 is attached to a substrate 110. Gold wires 130 are used to make connection between bonding pads 121 of the capacitive fingerprint chip 120 and a lead frame 111 on substrate 110. After chip attachment and wire bonding, the whole chip is molded with a high dielectric filler 140. However, during molding process, the difference of thermal expansion coefficient between the capacitive fingerprint chip 120 and the filler 140 will cause die mark or warpage, as the height difference H shown in FIG. 1. Besides, the filler 140 mixed with high dielectric compound has particle size in several μm range that cause surface roughness of the filler 140 greater than 2 μm and reduce sensing sensitivity. Furthermore, the wire bond loop section (as dashed circle shown in FIG. 1) is more susceptible to electrostatic discharge (ESD) breakdown.
Accordingly, it is highly desirable to provide a capacitive fingerprint sensor having a flat surface with wear resistance and better ESD toleration.